The most difficult requirement for far target location (FTL) is the determination of azimuth heading and inclination, with azimuth the most challenging. Currently, man-portable FTL systems employ a digital magnetic compass (DMC). However, the azimuth accuracy of DMCs is susceptible to errors from the magnetic effects of nearby objects (such as military vehicles), variations in the earth's magnetic field, and improper calibration. Therefore, non-magnetic methods are sought to measure azimuth that are also compact, low power, and economical.
Azimuth heading can be obtained by sensing the earth's rotation using north-finding inertial systems. Gyroscopes with performance high enough to provide the required degree of accuracy for FTL, however, tend to be too bulky and expensive for hand-held applications. MEMS-based gyros are small and inexpensive, but at present can only provide heading accuracies in the few degree range, far from the desired accuracy of ±1 mil. Optically based inertial sensing technologies such as fiber optic gyros (FOGs) and ring laser gyros (RLGs) are capable of the required level of accuracy for azimuth heading measurement, but are large and costly.